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In the inverter-fed motor drive system, inverter output-current distortion is reduced by enhancing the switching frequency and the motor torque ripples; magnetic noise is also reduced as a consequence. However, the system efficiency is also reduced due to the switching loss; this, in turn, sets a limit on any further enhancement of the switching frequency. This is particularly the case for the IGBT three-level inverter, since it is used in high switching frequency applications: the number of switching devices is twice that of the two-level inverter, thus the switching loss problem is more serious. The authors describe a new PWM strategy for the three-level inverter with an ability to reduce switching loss. In comparison with conventional sinusoidal PWM methods generating almost the same distorted load current, every phase of the inverter can have its switching stopped for 120° in one period by this new method. According to the load power factor angle β, if β ≤ 30° the ‘no switching duration’ can be set just in the centre of the peak value region of the load current, hence the switching loss can be greatly reduced. The reduction of switching loss is not dependent on β.
Kaku et al. (1997) studied this question.